A spectrometry system in general includes an ion source for ionizing analytes of a sample of interest, an ion analyzer for separating the analyte ions based on a discriminating attribute, an ion detector for counting the separated ions, and electronics (including, for example, a data acquisition system) for processing output signals from the ion detector as needed to produce user-interpretable spectral information. The spectral information may be utilized to determine the molecular structures of components of the sample, thereby enabling the sample to be qualitatively and quantitatively characterized.
In a mass spectrometry (MS) system, the ion analyzer is a mass analyzer that separates the ions based on their differing mass-to-charge ratios (or m/z ratios, or more simply “masses”). Depending on design, the mass analyzer may separate ions by utilizing electric and/or magnetic fields, or a time-of-flight tube in the case of a time-of-flight mass spectrometer (TOFMS) that determines the m/z ratio of an analyte ion by measuring its arrival time at the ion detector. The ion detector receives the separated ions and outputs measurement signals to electronics configured for processing the signals as needed to produce a mass spectrum. The mass spectrum is typically presented as a plot containing a series of peaks indicative of the relative abundances of detected ions as a function of their m/z ratios.
In an ion mobility spectrometry (IMS) system, the ion analyzer is a drift cell that separates ions based on their different collision cross-sections. In low-field drift-time IMS techniques, ions are pulled through the drift cell by a DC voltage gradient in the presence of a drift gas. Ions of differing collision cross-sections have differing mobilities through the gas environment and hence arrive at the ion detector at different drift times. The ion detector receives the separated ions and outputs measurement signals to electronics configured for processing the signals as needed to produce a drift spectrum. The drift spectrum is typically presented as a plot containing a series of peaks indicative of the relative abundances of detected ions as a function of their drift time through the drift cell.
In some spectrometry systems referred to as ion mobility-mass spectrometry systems (IM-MS), an IM drift cell is coupled with a mass analyzer to provide unique two-dimensional information about an analyte under investigation. Additionally, in some MS, IMS, and IM-MS systems, the sample supplied to the ionization apparatus may first be subjected to a form of analytical separation such as, for example, liquid chromatography (LC) or gas chromatography (GC). In such cases, the output of the LC or GC column (chromatographically separated analytes of the sample) may be transferred into the ionization apparatus through appropriate interface hardware.
To achieve high resolution and ion measurement accuracy, as an example, the timing of narrow output pulses (e.g., 400 picoseconds (ps) to 3 nanoseconds (ns)) from the ion detector must be determined within a very narrow timing window (e.g., 100 ps). The ion detector may not offer intrinsic blocking of the high voltages associated with the operation of the ion accelerator and ion detector. The output pulses from such a detector may have a direct current (DC) offset on the order of kilovolts (kV) from ground, for example 15 kV from ground. To enable the data acquisition system that receives the output from the detector to operate at a potential that is close to ground, an alternating current (AC) coupler may be coupled between the output of the ion detector and the input of the data acquisition system. The AC coupler rejects or filters the DC component of the signal, effectively normalizing the output pulses to a zero mean or a DC offset of zero. In addition to preserving the critical timing information embodied in the detector output pulses, the AC coupler must also maintain amplitude accuracy over a potentially wide range (e.g., 5 millivolts (mV) to 5 V) for accurate quantitation.
An AC coupler known in the art consists of two 50-Ohm (Ω) Subminiature version A (SMA) coaxial radio frequency (RF) connectors configured for threaded coupling to standard coaxial RF cables. The center conductors of these input and output SMA connectors are connected through a pair of coupling capacitors (also known as DC-blocking capacitors) in series, while the outer shells of the SMA connectors are connected through a similar pair of coupling capacitors in series. This known AC coupler also includes high-value resistors shunting the capacitors to equalize the DC voltage drops across the capacitors, allowing capacitors with lower voltage ratings to withstand the applied DC voltage. These resistors have no effect on the desired signals passing through the AC coupler. The known AC coupler may handle the DC common mode offset and the low-frequency components of the normal mode signal adequately. However, the known AC coupler may not transmit the high-frequency components accurately, resulting in distorted pulses, extra pulses, or ringing waveforms, depending on the specifics of the connections to the MS system. Anomalies that the data analysis software resolves as separate peaks will be erroneously identified as ions that are not actually present in the sample under analysis. Peak broadening degrades mass resolution, possibly hiding smaller nearby peaks entirely. Extended tails appended to peaks shift the effective baseline, introducing nonlinearity in sample quantitation. Typical approaches taken to compensate for ringing may also shift the assumed baseline, with similar effects.
The connectors, capacitors, interconnecting conductors, and dielectric materials that make up the AC coupler form a transmission line through which the signals pass. Each region of uniform geometry along the length of the AC coupler has a specific characteristic impedance. A signal that is incident on a transition from one geometry to another is split into a transmitted component and a reflected component. Multiple reflections can occur as a signal passes through the AC coupler, separated in time by the transmission delays between the impedance transitions. These reflections should be minimized in order to preserve the critical timing and amplitude properties needed for correct interpretation of the signal received by the data acquisition system.
Certain features of conventional AC couplers are likely to promote undesired reflections. One feature is abrupt turns in the signal path, such as a 90-degree turn in passing from the input coaxial connector to the plane of the capacitors, followed by another 90-degree turn in passing from the plane of the capacitors to the output coaxial connector. Another feature is abrupt transitions in the sizes of interconnected electrical conductors. For example, the center conductor of a coaxial connector may be 1 millimeter (mm) in diameter and directly connected to a signal trace that is 10 mm in width. Another feature is the direct connection between the unbalanced input coaxial connector and output coaxial connector to the symmetrical parallel-plate transmission line formed by the pairs of coupling capacitors without intervening balun transformers. This exposes the surrounding structures to the common mode component of the signal, which sees a different impedance than the normal mode component and propagates at a different velocity. These effects are dependent on both frequency and system layout, leading to uncontrolled signal distortion.
More generally, deficiencies in the configuration or construction of known AC couplers may lead to a number of problems. These problems include poor signal fidelity and thus reduced mass resolution and less effective quantitation, insufficient wideband impedance matching, sensitivity to nearby objects due to an unshielded balanced transmission line structure, the occurrence of spurious pulses leading to false ion identification, ringing and thus noisy baselines and poor linearity, unstable impedance, and lack of adjustability of impedance.
Therefore, there is an ongoing need for AC couplers that address problems such as those discussed above. There is also a need for AC couplers that provide improved impedance matched, high-voltage, wideband performance for transmitting AC signals while blocking DC offset voltages.